1. Field of the Invention
The present invention relates generally to the art railway signaling systems and more particularly to relays and solid-state interlocking systems.
2. Description of Related Art
Present day railway signaling systems normally employ relay interlocking, solid-state interlocking, or a combination of both.
FIG. 1 A illustrates an early signaling system in which a number of relays, 1a-1i, are individually dedicated to performing multiple functions. Each relay circuit is wired to a track circuit and employs contacts to determine particular track conditions, such as whether there was a train on that section of track. Based upon the circuit design and whether or not certain contacts were open or closed each relay carries out the particular function to which it is dedicated. In this way a plurality of relays comprised the earliest interlocking railway signaling system. In this traditional relay system, multiple specially designed relays circuits each perform a particular function. A different relay circuit is provided for each function to be performed at a certain location. The overall system consists basically of multiple relays implementing multiple functions. A big disadvantage of this system is that the relay logic is fixed by the circuit design. If you want to add a new function, or modify an existing one, you must re-wire the circuit. This can entail adding new circuits and contacts, as well as adding additional relays. Since each different function requires a different relay circuit design, a large number of different circuits and types of relays are required which can result in high costs. Furthermore, as new functions are conceived of, the complexity of the circuits becomes a problem, requiring more contacts, more wiring, and more relays. The result is not only higher costs but also an increased likelihood of crossed wires and shorted circuits.
The advent of multifunction solid-state interlocking systems brought the flexibility of programmable logic and the benefits of integrated monitoring and control of an entire system. A single multifunction central processor may oversee and carry out many of the functions previously performed by relays. FIG. 1B illustrates how a multifunction solid-state system 2 may be installed at a location. Generally, the multifunction system 2 may be wired into the track circuit and the contacts formerly connected to each relay 1. The multifunction system 2 thereby receives the inputs that formerly received by the relays 1. Based upon the inputs the multifunction system 2 performs the multiple functions once carried out individually by separate relays 1. The big advantage of the multifunction system 2 is that programmable logic may be employed to evaluate the inputs and implement functions which the relays 1 either could not perform at all or for which the complexity of the circuit that would be required made it impractical. Additionally, by interconnecting multiple multifunction systems 2, a central control center can monitor conditions along practically the entire line of track in operation. This overview permits the multifunction system 2 to organize, plan and prioritize each of the multiple functions. A hybrid signaling system wherein a multifunction solid-state system interacts with traditional relays is illustrated in FIG. 1C. In this system, which is the norm, a multifunction solid-state interlocking system 2 may retain and interact with traditional relays 1 as part of an overall interlocking signaling system. The multifunction system 2 generally operates in the same manner described above, except that the multifunction system 2 additionally receives inputs from relays 1. A difference is that some of the relays 1 may perform functions 3 independent of the multifunction system 2. These relays are typically vital relays which are employed in combination with the multifunction system 2 as safeguards against certain unsafe conditions. For example, a vital relay may be employed to ensure that if a malfunction occurs, the multifunction system 2 cannot initiate a signal or other device in the face of an oncoming vehicle.
Despite the advantages of solid-state systems, such an integrated logic control and multifunction performing system can have certain disadvantages. In the relay interlocking system, all of the functions and logic are segregated, thus simplifying maintenance and trouble-shooting of malfunctions. Additionally, new relay circuits can be added to perform additional functions without affecting the other relays in the rest of the system. In contrast, existing multifunction solid-state systems can be handicapped by the necessity of retesting the entire system after changes in interlocking logic or hardware is made, a typically time consuming and very expensive process. Recently developed methods of checking application logic to ensure that only the desired changes are made may not be fully satisfactory. Another problem that can be inherent in the solid-state interlocking system is latency. Because inter-processor communications are limited to a very small number of bits at sporadic intervals, latency in contemporary centralized processors can be difficult to predict before actually constructing and testing the system. Correction of latency-related problems often requires expensive redesign and additional hardware. Latency concerns may prevent consideration of real-time encoding and decoding functions for more than one or two I/O ports. Latency frequently may require disadvantageous circuit designs employing external relays or contacts, which may increase disproportionately with I/O capacity, posing additional burdens on systems with limited processor capacity. Reliability can be another problem area which may be improved upon in contemporary multifunction solid-state systems. Generally, multifunction solid-state interlocking systems have no redundancies. Failure of any individual output, component failure, or short circuit may in some instances cause the entire system to shut down. To obtain redundancy in these systems may require two identical sets of hardware along with some sort of fail-over mechanism between them.
From an economic standpoint, conventional solid-state systems invariably have step-function cost/size relationships, limited maximum capacity and limited facilities for integration of standby hardware, data loggers, etc. Large programmable multifunction processor systems can be too costly and complex for end-of-sidings and too limited in size and speed for large rapid-transit interlockings. It is these E-O-S and rapid-transit applications where the economic benefit of solid-state interlocking is greatest because of the physical size of equipment or volume of business. For small interlockings, various subspecies of programmable multifunction systems have evolved. However, limited production volume of these smaller solid-state systems and their ever-changing configurations are an unwelcome challenge for those who must design and support these products. Elsewhere, large solid-state interlocking systems have become overly complex and costly due to system capacity limitations and the continuing need for vital relays to isolate noise or arbitrate between normal and standby outputs.
Because of the numerous disadvantages inherent in current interlocking railway signaling systems, there has arisen a need for improving the contemporary multifunction interlocking systems. Moreover, there is also a need for an economical means to upgrade older, obsolete relay interlocking systems in situations where there may not be enough economic incentive to completely convert to a multifunction solid-state interlocking system.